A new technology has been reported that involves protein engineering to obtain single chain antibodies (fluorogen activating peptides, or FAPs) that bind engineered chromophores (Fluorogens) that are not fluorescent in aqueous solution but which become highly fluorescent when bound to a corresponding FAP. See, Szent-Gyorgyi C. et al., Fluorogen-activating single-chain antibodies for imaging cell surface proteins, Nat Biotechnol, 26:235-240, (2008); Patent Application WO 2008/092041 A2; and U.S. Published Application US2011/0159519 A1, the contents of each of which are hereby incorporated by reference herein. Various FAP-Fluorogen combinations have been used to detect the location of cellular proteins that have been genetically labeled with FAPs. Szent-Gyorgyi, et al., Nat Biotechnol. 26:235-240, (2008). Also FAP-Fluorogen combinations have been developed that allow detection of pH and calcium concentrations in the local environment of the protein labeled with a FAP. See, Grover A. et al, Genetically Encoded pH Sensor for Tracking Surface Proteins through Endocytosis, Angewante Chemie, published online: 29 Mar. 2012. This technology has been used in biological assays in drug discovery and basic research to track membrane surface proteins that are exposed to agonist and antagonist molecules. See, Wu Y. et al., Discovery of regulators of receptor internalization by high throughput cytometry, Mol. Pharmacol. (2012) PMID: 22767611.
At present, the longest wavelength of fluoromodule excitation/emission maxima has been 633/667 nm. This partially limits the application of the FAP technology, particularly towards the interrogation of tissue slices, organs, and whole animals, which absorb and scatter visible light. Further, conventional fluorescence-based drug discovery assays are sometimes confounded by various compounds present in most drug candidate libraries that are themselves fluorescent when the assay is carried out. There are very few such drug candidates that fluoresce at long wavelengths, especially in the Near Infrared Region (NIR).